1. Field of the Invention
This invention relates generally to filters and, more particularly, is directed to a multi-purpose filter.
2. Description of the Prior Art
It may be desirable for a filter to provide different filter characteristics at different times. For example, it may be desirable to provide a low pass filter characteristic at one time, a band pass filter characteristic at another time and a trap filter characteristic at still another time. In this regard, a prior art multi-purpose filter is known which is formed from a plurality of series-connected operational amplifier circuits which function as integration circuits. By selectively combining the input signal supplied to the multi-purpose filter and/or various output signals from the different operational amplifiers, different filter characteristics can be obtained.
However, such a prior art multi-purpose filter requires the use of four operational amplifiers which have discrete resistor and capacitor elements connected therewith and, as a result, is relatively complicated in construction. Further, the characteristics of such multi-purpose filter are determined, at least in part, by the characteristics of each integration circuit comprised of the operational amplifiers and the values of the resistors and capacitors connected therewith. If the temperature coefficients of these elements are large, that is, if these elements have a large temperature dependency, the multi-purpose filter has a temperature characteristic which is not negligible. Thus, the feedback resistors and capacitors must be carefully chosen to have a small temperature dependency which, in turn, results in an increased cost of the filter.